1. Field of the Invention
The present invention relates to digital printing devices, and, in particular, to methods and apparatus for generating black, or K, separation screens for use in halftoning operations that convert a color input to a binary output suitable for printing.
2. Description of the Related Art
Most computer-driven printing devices which generate hard copy, such as laser, dot-matrix and ink-jet printers, print in a binary fashion--the output medium is divided into an array of picture elements or "pixels" and the devices can either print a small colored dot at each pixel location or leave the pixel location blank. In the case of monochrome printers, all of the dots are printed with a single color whereas with color printers a dot color is chosen from a small set of colors. In any case, the dot itself has a uniform color so that the resulting output consists of an array of colored and blank pixels.
Pictorial images such as those produced by photographic techniques or by computerized imaging systems, by contrast, are continuous in tonality. If a monochrome image is divided into pixels, each pixel exhibits a "grayscale" color whose tonal value falls within a range of tonal values. Similarly, if a color image is divided into pixels, each pixel exhibits a hue and an intensity both of which fall in ranges. In order to reproduce such "continuous-tone" images by means of electronic printing, the images must therefore be converted into a form which is suited to the characteristics of the printing device, generally a binary format. This conversion process, which may take many forms, is generically referred to as "halftoning." Although a halftone image actually consists solely of a spatial pattern of binary pixels (colored or blank dots), the human visual system integrates this pattern to create an illusion of a continuous-tone image.
During the printing process, the image to be printed is divided into a series of pixels and the value of the image in each pixel is quantized to produce a multi-bit digital word which represents the tonal value of the pixel. The image is thus converted to a stream of digital words which are provided to the printing device. In order to convert the format of each word into a format suitable for reproduction on the digital device, halftoning is performed on the digital word stream during a process called preprocessing. Numerous halftoning techniques have been developed and refined over the years. In their simplest form, such techniques compare the value of each digital word with a threshold level, and generate a binary output pixel value depending on the relative values.
For example, a digital scanner processing a continuous-tone monochrome image might generate a stream of multi-bit words representing the detected light intensities. Commonly, the numerical value of these words ranges from 0 to 255, corresponding to a 256-level gray scale or an eight-bit word. If such a digital word stream is to be reproduced on a binary printing device, the halftoning process compares the scanner output words with a either a single threshold value or an array of threshold values to produce the required binary output pixel stream. In such a system, each 8-bit scanner word has effectively been compressed into a single-bit output word.
Color images are typically processed by separating each color into one or more color components or "primaries" whose superposition generates the desired color. Generally, three primary colors (either the conventional "additive" primary colors--red, green and blue or the conventional "subtractive" primary colors--cyan, magenta and yellow) are used. A digital scanner processing a continuous-tone color image might generate a stream of multi-bit words for each of the three color components (usually the additive primary colors). Commonly, the numerical value of these words also ranges from 0 to 255, corresponding to 256 intensity levels or an eight-bit word. Thus, each colored pixel is represented by 3 eight-bit words or 24 bits total.
The digital word stream corresponding to a colored image is halftoned by comparing the eight-bit word for each color component with a threshold value in the same manner as monochrome processing. The color components are processed separately so that the three 8-bit scanner words are compressed into a three-bit output word which is eventually printed as three dots--each dot being printed in one of the primary colors.
Theoretically, it is possible to reproduce the full range of printable colors using varying intensities of the three primary colors and superimposing the three dots. For example, if the maximum intensity of each color is printed and the three dots are superimposed, the resulting dot should appear as black. In practice, however, superimposing or overprinting 100% of each primary color produces a gray-brown colored dot and completely saturates the paper with ink so generally black is added as a fourth color. Thus, the colors used are red, green, blue and black (RGBK) or cyan, magenta, yellow and black (CMYK).
In reproducing graphic images with traditional printing presses, the halftoning operation is performed by photographing an image through a halftone screen. A halftone screen is a grid defined by two patterns of regularly-spaced parallel opaque lines which cross at 90.degree. angles. The crossed lines form rows of square "holes" in the screen. The spacing of the lines on the halftone screen, measured in lines per inch, is known as the screen "frequency."
Although the screen frequency is constant, during the photographic process, the holes in the screen act like pinhole lenses producing dots on the photographic negative whose size is proportional to the amount of light reflected from the original image. Consequently, relatively light areas of the image produce larger black areas on the negative than darker areas. When the negative is reproduced as a positive the larger dark areas translate into smaller dots. The viewer's eye perceives the dot patterns as gray areas.
When such printing presses are used to print a color image, color "separations" of the image are made by photographing the original image four separate times through the same halftone screen but with a different color filter for each primary color and black. The result is four separate halftone dot patterns. However, if the dot patterns are simply superimposed on printing, the dots where three dot colors overprint result in color distortions. In addition, it is generally impossible to obtain perfect registration or alignment of the dot patterns on a conventional printing press and, if the dots do not overprint exactly, additional color distortions arise
Consequently, in order to avoid color distortions in traditional printing arrangements, the dots from the patterns are not overprinted, but are instead printed in close proximity so that the viewer's eye integrates the dots to provide the shades of color in the original image.
More specifically, each halftone screen corresponding to a particular color is rotated relative to the position of the other screens causing the resulting dot patterns to be rotated at different angles. In accordance with conventional terminology, the direction of printing is called the printing or pixel grid "direction." The halftone dots of each color all lie along a screen direction which, when the pattern is rotated, forms an angle with the printing direction called a screen "angle." When four color printing is used, each color is printed in different screen angle and a conventional printing pattern separates the screen angles by 30.degree. in order to produce the best looking halftoned image.
Since, as previously mentioned, the screens consist of lines running at 90.degree. angles, there is room for only three 30.degree. rotations before the one of the patterns overprints another, thus, in four color printing, one color is printed at a "odd" angle relative to the 30.degree. spacing. The traditional screen angles for CMYK printing are 45.degree. for black, 75.degree. (45.degree.+30.degree.) for magenta, 105.degree. (75.degree.+30.degree.) for cyan and 90.degree. for yellow. The darkest color (black) is printed at a 45.degree. angle because dot patterns printed at 45.degree. have been found to produce the least noticeable artifacts, whereas yellow, the color that is printed at the odd angle of 90.degree., is the lightest color and has been found to be the least likely to make visible artifact patterns. The result of the screen angle differences is a circular "rosette" pattern of color dots that provides the illusion of full color images when viewed at a distance from the paper.
The problem with using the conventional screen angles is that slight variations in the exact angles produce geometrical artifact patterns called moire patterns. As the screen angles differ more and more from the theoretical values, the moire patterns become more visible and eventually overwhelm the image.
The moire pattern problem is especially acute on digital images produced by "raster" devices, such as printers or digital imagesetters, which devices can only print a fixed pattern of dots of equal size. For example, a typical printer cannot vary either the size or position of each pixel, all it can do is print one or more dots at a predetermined grid location or leave the location blank. Thus, the only halftone dot spacings (1/(dot spacing) along the screen direction is called the screen frequency) that are possible on such a device are those that divide evenly into the device resolution, since the device cannot print part of a pixel.
These digital devices can simulate traditional halftone dots by grouping the pixels into patterns of halftone "cells" where each cell corresponds to a single halftone dot. However, the above-mentioned moire pattern problem occurs when the halftone cells are rotated to produce color separations, since the corners of the halftone cell must fall in the pixel grid. Thus, when the traditional screen rotation approach is used, there are a limited number of angles available for rotating the halftone cells and these angles may be close, but are not equal to, the theoretical angles.
Further, the traditional screen rotation approach does not address another problem which occurs in some high-resolution desktop printers in which the dots are oversized. For example, in a printer which has a resolution of 720 dpi (dots per inch), it is desirable that the dot diameter be slightly greater than the dimension of a square pixel grid so that the dots slightly overlap when printed adjacent to each other. However, many such printers actually print dots which are considerably larger than the desired dot size. In these latter printers, if a black dot is printed adjacent to a colored dot, the black dot (which is essentially opaque) overprints the colored dot. This overprinting causes the entire image to appear darker than the corresponding original image.
In order to reduce the visibility of the moire patterns, a different type of screen was developed by Marcel Coderch of Anaya Systems and is known as a "Flamenco" screen. In the Flamenco screen, rather than rotating the color screens relative to each other, all color dots are located on a fixed grid with optimal spacing of the dot centers. The screen angle for each color (including black is 45.degree.). In order to separate the dots, the color screens are offset slightly from one another in the horizontal and vertical directions causing the dots to form a repeating square pattern with a different color dot located on each corner of the square pattern.
The Flamenco screen also reduces the overprinting problem caused by excessive dot overlap because, in the Flamenco pattern, the black dots are physically separated from the other colored dots and, thus, the halftoned colors retain much of their original brightness even in cases of severe dot overlap.
Accordingly, it is an object of the present invention to provide a separation screen pattern which effectively separates the black dots from the color dots.
It is another object of the present invention to produce a separation screen pattern which does not require precise angular adjustments to separate the black and colored dots.
It is a further object of the present invention to produce a separation screen which can be used with a raster device.
It is yet another object of the present invention to produce a separation screen which can be quickly and economically generated using standard techniques.